A fuel cell has been noted as a next-generation electricity-generating device which can contribute to the solution to environmental issue and energy issue, which have been recently socially great problems, because it exhibits a high electricity generating efficiency and excellent environmental properties.
Fuel cells are normally divided into several types by the kind of electrolyte, and among these types, the polymer electrolyte fuel cell (hereinafter occasionally referred to as “PEFC”) has a small size and a high output as compared with any other types of fuel cells and has been regarded as a next-generation mainstream of electric supply such as small-scale on-site electric supply, electric supply for mobile body such as power source for vehicle and electric supply for portable devices.
Thus, PEFC has excellent advantages in principle and has been extensively developed for practical use. This PEFC normally uses hydrogen as a fuel. Hydrogen decomposes proton (hydrogen ion) and electron in the presence of a catalyst disposed on the anode side of PEFC. Among these components, the electron is supplied to the exterior where it is then used as electricity and then circulated to the cathode side of PEFC. On the other hand, the proton is supplied to a proton conducting membrane (electrolyte membrane) through which it moves to the cathode side. On the cathode side, the proton, the electron which has been circulated and oxygen which has been introduced from the exterior are combined in the presence of a catalyst to produce water. In other words, as viewed singly, PEFC is a very clean energy source which allows electricity to be drawn when water is produced from hydrogen and oxygen.
As the fuel for fuel cell there is normally used hydrogen, but a fuel cell which directly introduces a fuel other than hydrogen such as alcohol, ether and hydrocarbon thereinto so that proton and electron are drawn from such a fuel in the presence of a catalyst has been extensively studied. A representative example of such a fuel cell is a direct methanol fuel cell (hereinafter occasionally referred to as “DMFC”) which uses methanol (normally in the form of aqueous solution) as a fuel.
Herein, the proton conducting membrane acts to transmit proton produced on the anode to the cathode side. As mentioned above, the movement of proton occurs in cooperation with the flow of electron. In other words, in order that PEFC might give a high output, i.e., high current density, it is necessary that protonic conduction be conducted at a high rate in a sufficient amount. Accordingly, it is no exaggeration to say that the performance of the proton conducting membrane is a key material that determines the performance of PEFC. Further, the proton conducting membrane acts to not only conduct proton but also play a role as an insulating membrane that electrically insulates the anode off the cathode and a fuel barrier membrane that prevents the fuel supplied to the anode side from leaking to the cathode side.
The main proton conducting membrane which is now used in PEFC is a fluororesin-based membrane comprising a perfluoroalkylene as a main chain and partly having a sulfonic acid group at the end of perfluorovinylether side chain. As such sulfonated fluororesin-based membranes there are known, e.g., Nafion (trade name) membrane (Du Pont Inc.; see Patent Reference 1), Dow membrane (Dow Chemical Inc.; see Patent Reference 2), Aciplex (trade name) membrane (Asahi Kasei Corporation; see Patent Reference 3), Flemion (trade name) membrane (ASAHI GLASS COMPANY), etc.
It is said that these fluororesin-based membranes have a glass transition temperature (Tg) in the vicinity of 130° C. under wet conditions where fuel cells are used, and in the vicinity of this temperature, so-called creep occurs, resulting in the change of the structure of the proton conducting membrane in the membrane and hence making it impossible for the proton conducting membrane to exhibit a stable protonic conductivity. Further, the member degenerates to swollen state and becomes a jelly-like material that can easily be destroyed to cause failure in the fuel cell.
For the aforementioned reasons, the maximum temperature of present use at which the fuel cell can be stably used over an extended period of time is normally regarded as 80° C.
In its principle, a fuel cell uses chemical reaction and thus exhibits a higher energy efficiency when operated at high temperatures. In other words, as viewed on the basis of the same output, a device which can be operated at high temperatures can be reduced more in size and weight. Further, when the fuel cell is operated at high temperatures, its exhaust heat, too, can be utilized, allowing so-called cogeneration (combined supply of heat and electricity) that drastically enhances the total energy efficiency. Accordingly, it is considered that the operating temperature of a fuel cell is somewhat higher, normally 100° C. or more, particularly preferably 120° C. or more.
Further, in the case where hydrogen which has not been thoroughly purified is supplied into a fuel cell, a phenomenon that the catalyst used on the anode side can be deactivated by impurities (e.g., carbon monoxide) in the fuel, i.e., so-called catalyst poisoning occurs, raising a great problem that governs the life of PEFC. It is known that this catalyst poisoning, too, can be avoided when the fuel cell can be operated at high temperatures, and in this respect, too, it is preferred that the fuel cell be operated at higher temperatures. Further, when the fuel cell can be operated at higher temperatures, there is no necessity of using a purified product of noble metal such as platinum, which has heretofore been used, as the catalyst, making it possible to use an alloy of various metals to great advantage from the standpoint of cost or resources.
On the other hand, direct fuel type fuel cells which operate by using directly a fuel other than hydrogen such as DMFC are now under studies of efficient extraction of proton and electron from the fuel, but the enhancement of fuel barrier properties of the proton conducting membrane and the high temperature operation for efficient action of catalyst are considered technical assignments to be solved to obtain a sufficient output.
Thus, although it is considered desirable from various standpoints of view that PEFC be operated at higher temperatures, the heat resistance of the proton conducting membrane is up to 80° C. as previously mentioned and the operating temperature of the fuel cell, too, is thus limited to 80° C. at present.
By the way, the reaction occurring during the operation of the fuel cell is an exothermic reaction, and when the fuel cell is operated, the temperature in PEFC then spontaneously rises. However, since Nafion, which is a representative proton conducting membrane of present use, has so heat-resistant as to withstand 80° C. at highest, it is necessary that PEFC be cooled so that the temperature thereof doesn't rise to 80° C. or more. Cooling is normally accomplished by water cooling, and PEFC is devised at the separator portion thereof for cooling. When provided with such cooling means, PEFC is large-sized and heavy as a whole, making it impossible to make sufficient use of small size and light weight, which are inherent characteristics of PEFC. In particular, when the critical operating temperature is 80° C., effective cooling is made difficult in the water cooling system, which is the simplest cooling means. When operation can be made at 100° C. or more, the heat can be utilized to evaporate water, making effective cooling, and the circulation of water makes it possible to drastically reduce the amount of water to be used during cooling and hence attain the reduction of size and weight of the device. Since the comparison of temperature control at 100° C. or more with temperature control at 80° C. in the case where the fuel cell is used as an energy source for vehicle shows that the capacity of radiator and cooling water can be drastically reduced, PEFC which can be operated 100° C. or more, i.e., proton conducting membrane having a heat resistance to 100° C. or more has been keenly desired.
Although PEFC has been required to operate at high temperatures, that is, proton conducting membranes are required to have high temperature resistance from various standpoints of view such as electricity generating efficiency, cogeneration efficiency, cost, resources and cooling efficiency, no proton conducting membranes having both sufficient protonic conductivity and heat resistance exist.
Under these circumstances, in order to raise the operating temperature of PEFC, various heat-resistant proton conducting materials have been studied and proposed to date.
A representative example of these heat-resistant proton conducting materials is a heat-resistant aromatic polymer material that substitutes for the conventional fluorine-based membranes, and examples of such a heat-resistant aromatic polymer material include polybenzimidazoles (see Patent Reference 4), polyethersulfones (see Patent References 5, 6), polyether ether ketones (see Patent Reference 7), etc.
These aromatic polymer materials are advantageous in that they undergo little structural change at high temperatures, but on the other hand, most of them have sulfonic acid groups, carboxylic acid groups, etc. incorporated directly in the aromatic group, and in this case, they can undergo remarkable desulfonation or decarbonation at high temperatures and thus are not suitable for high temperature-working membrane.
Further, when water exists and at high temperatures, the entire membrane tends to swell remarkably as the fluororesin-based membrane does, and due to the change of the size of the membrane, stress is applied to the junction of the membrane-electrode assembly, making it very likely that the membrane and the electrode can be exfoliated at the junction or the membrane can be broken, and there rises a problem that the reduction of strength of the membrane due to swelling can cause membrane destruction. Further, since all these aromatic polymer material are polymer compounds which stay rigid when dried, there rises a problem that the membrane can undergo destruction or the like during the formation of membrane-electrode assembly.
In order to solve these problems, a method has been studied which comprises incorporating these electrolytes in a porous resin (see Patent Reference 8). In this case, membrane strength and dimensional stability can be greatly improved, but the proton conducting material used is similar to the conventional materials and leaves something to be desired in essential thermal stability.
On the other hand, the following inorganic materials have been proposed as proton conducting material. For example, Minami et al obtained proton conducting materials by adding various acids to hydrolyzable silyl compounds (see Non-patent Reference 1). These inorganic materials are stable even at high temperatures but are disadvantageous in that these acids are scattered and lost to lower the conductivity after prolonged use because there are present many acids which are not connected to crosslinking groups.
In order to overcome these problems, a method which comprises grinding a proton conducting inorganic material, and then mixing the material thus ground with an elastomer (see Patent Reference 9), a method which comprises mixing the material thus ground with a sulfonic acid group-containing polymer (see Patent Reference 10), etc. for example have been attempted, but since these methods only involve the mixing of a polymer material as a binder with an inorganic crosslinked material, the mixture has no great difference in basic thermal properties from polymer material alone and thus undergoes structural change of polymer material at high temperatures and doesn't exhibit stable protonic conductivity and high protonic conductivity in many cases.
Although various electrolyte membrane materials have been researched and developed to eliminate the problems with the conventional polymer electrolyte fuel cells as mentioned above, no proton conducting membranes having sufficient durability at high temperatures (e.g., 100° C. or more) and satisfying various desired physical properties such as mechanical strength have ever existed to date.
On the other hand, in DMFC, which uses methanol as a fuel instead of hydrogen, methanol is brought into direct contact with the membrane. Sulfonated fluororesin-based membranes which are used at present exhibit a high affinity for methanol, and when the membrane absorbs methanol, it extremely swells and, in some cases, dissolves, causing failure in the fuel cell.
For example, Nafion membrane, which is a representative example of fluororesin membranes, is a flexible membrane which is rigid when dried but swells great when wet. Thus, when the dimension of the membrane greatly differs from when dried to when wet, MEA can be difficultly produced and the membrane always undergoes elongation/shrinkage with the change of temperature and humidity in the fuel cell caused by the change of operating conditions even during the operation of the fuel cell, making it likely that the breakage of the membrane or the destruction of MEA can occur. Further, when swollen, the membrane becomes weakened, there arises a risk of not only the aforementioned dimensional change but also membrane breakage in the case of occurrence of pressure difference in the fuel cell.
Further, when this fluororesin membrane is exposed to a temperature as high as, e.g., 150° C. while being wet over an extended period of time, the membrane itself collapses in the form of jelly and thus can no longer be used as a proton conducting membrane for fuel cell. Further, even when exposed to a temperature of about 120° C., the fluororesin membrane undergoes creep that leads to degeneration to swollen state. Once degenerated, the fluororesin membrane becomes hard and brittle when dried due to the change of operating conditions of fuel cell, making it likely that the breakage or cracking of the membrane and even the destruction of MEA can occur. This similarly occurs with membranes having an aromatic molecular structure in its main chain.
Further, methanol leaks to the oxygen electrode side, causing drastic drop of the output of the fuel cell. This a problem that occurs also with an electrolyte membrane containing an aromatic ring. Therefore, no efficient and durable membranes exist at present also for DMFC.
The aforementioned Nafion membrane, which is now used as a proton conducting membrane on a standard basis, has a polytetra(or tri)fluoroethylne in its main chain and a sulfonic acid group in its side chains. Since the polytetra(or tri)fluoroethylne is nonpolar and water-repellent and the sulfonic acid group in the side chains is polar and hydrophilic, a phase-separated structure is spontaneously formed, resulting in the formation of a structure in which a sulfonic acid group is accumulated in a high concentration, which structure acting as a proton conducting channel (see Non-patent Reference 2).
On the other hand, most of the aromatic hydrocarbon-based membranes which are now under studies as heat-resistant membrane have such a phase structure (see Non-patent Reference 3). As a result, a high conductivity cannot be obtained unless a large amount of acid is uniformly present in the membrane, but the incorporation of a large amount of acid occasionally causes the membrane to have deteriorated water resistance that renders the membrane soluble in water or highly swellable at high temperatures.    (Patent Reference 1) U.S. Pat. No. 3,282,875    (Patent Reference 2) JP-A-4-366137    (Patent Reference 3) JP-A-6-342665    (Patent Reference 4) JP-A-9-110982    (Patent Reference 5) JP-A-10-21943    (Patent Reference 6) JP-A-10-45913    (Patent Reference 7) JP-A-9-87510    (Patent Reference 8) U.S. Pat. No. 6,242,135    (Patent Reference 9) JP-A-8-249923    (Patent Reference 10) JP-A-10-69817    (Non-patent Reference 1) Solid State Ionics, vol. 74, page 105, 1994    (Non-patent Reference 2) Journal of Polymer Science, Polymer Physics, vol. 19, page 1,687, 1981    (Non-patent Reference 3) MEMBRANE, vol. 28, page 14, 2003
The present invention has been worked out in the light of the aforementioned circumstances and an object of the present invention is solve the problems with conventional polymer electrolyte fuel cells and provide a proton conducting membrane which exhibits an excellent protonic conductivity and a high heat resistance and dimensional stability even at high temperatures, a method for producing the same and a fuel cell using the same.
Further, another object of the present invention is to provide a proton conducting membrane excellent in durability, fuel barrier properties, etc., a method for producing the same and a fuel cell which can cope with high temperature operation or direct supply of a fuel such as methanol by using the same.